The present invention relates to removal of organic pollutants from aqueous effluent streams.
In many industries, eg the chemical and nuclear industries, considerable volumes of aqueous effluent are produced which require treatment to minimise the concentrations of pollutants before discharge to the environment. One class of pollutants in particular which require removal are organic compounds. One way of removing organic compounds is by oxidising the compounds, eg to CO2.
Destructively oxidising organic compounds in aqueous streams can be achieved in a number of ways. Steam stripping may be used initially to remove volatile compounds from aqueous streams. The compounds may then be destroyed in the gas phase.
Although volatile compounds can be treated in this way, non-volatile compounds remain in the liquid effluent and must be treated by a different technology.
This may be achieved by processes such as non-catalytic or catalytic wet air oxidation in which the effluent is heated, eg above 200xc2x0 C., under elevated pressure so that the system remains liquid. The need to maintain the effluent at high temperature and pressure is obviously undesirable. Also the typical limits on the concentration of organic compounds in solution which can be treated by the method are in the range 2-20%. Therefore, other methods must be found to treat effluents with lower concentration levels than this.
Lower temperature oxidation of organic compounds in aqueous solutions can be achieved through the use of catalysts. To date, numerous homogenous catalyst systems have been used. These have included, for example, mixtures of hydrogen peroxide and FeCl2 (Fenton""s reagent) or mixtures of hydrogen peroxide and chromium(VI) solutions.
However, problems can arise in such homogeneous catalyst systems. For example, since the catalyst is in the same phase as the solution, a proportion of the dissolved catalyst itself may be discharged to the environment along with the treated effluent leading to loss of valuable catalyst and accumulation of catalyst in the environment. Furthermore, although many homogeneous catalytic systems are generally efficient at organic removal at moderate concentrations, at low concentrations of pollutants, such as several ppm, their efficiency can be significantly reduced.
Heterogeneous catalysts, such as metals or metal oxides, are more commonly used to catalyse reactions of gas phase organic compounds and have been used less widely in the aqueous phase. Problems are likely to be encountered when using such catalysts to catalyse reactions in the aqueous phase. For example, such catalysts may dissolve in the aqueous solution. Also, contact between the organic compound and the catalyst may be less effective at low concentrations of the compound due to interfering effects of the water molecules. In the case of supported catalysts, the support material, such as silica or alumina, may become hydrolysed which further reduces the ability of the organic compound to contact the catalyst.
Nevertheless, titanium dioxide, TiO2, has been used in the photocatalytic oxidation of organic compounds in which ultraviolet light is used to excite electrons across the band gap in TiO2 and promote oxidation of the organic compound. However, the efficiency of these catalysts is low.
In view of the problems detailed above, it can be seen that the need exists for improved methods of organic destruction.
According to the present invention there is provided a method of destructively oxidising an organic compound present in an aqueous solution, the method comprising oxidising the organic compound in the presence of a catalyst which contains uranium.
Desirably, the catalyst comprises a uranium oxide with a stoichiometry from UO2 to UO3 inclusive. The catalyst may, for example, comprise UO2, U3O8, UO3 or another uranium oxide. The catalyst may comprise a mixture of two or more such uranium oxides. Preferably the catalyst comprises U3O8.
The catalyst may additionally contain one or more other metals. The one or more other metals may comprise, for example, vanadium, iron, copper or platinum.
The one or more other metals may be present as metal oxides, for example, vanadium, iron, copper or platinum oxides.
The catalyst may, for example. comprise a uranium oxide as defined above and one or more other metals. The one or more other metals may for example be vanadium, iron, copper or platinum or oxides thereof. A catalyst containing uranium and vanadium has been found to be particularly effective for destroying organic compounds in the present invention.
The catalyst may comprise a mixed metal oxide. The catalyst may comprise a mixture of single phase metal oxides.
The catalyst may comprise a mixed metal oxide which includes uranium and at least one other metal. The uranium-containing mixed metal oxide may be present with a uranium oxide and/or one or more other metals or metal oxides.
The catalyst may be supported on a support, for example, silica (SiO2), alumina (Al2O3), zeolites, activated carbon, titania (TiO2), zirconia (ZrO2) or ceria (CeO2).
The catalyst need not comprise a uranium oxide, but may comprise another uranium containing substance.
It has been found that oxides of uranium have a high catalytic activity for the oxidative decomposition of organic compounds in aqueous solutions. It has also been found that the activity may be enhanced in some cases by the incorporation of other metals or metal oxides in the catalyst. Different metals or metal oxides may have different effects on the activity of the catalyst for destroying a particular organic compound. One or more particular metals or metal oxides may be incorporated in the catalyst to enhance the destruction of any particular organic compound. Thus the composition and form of the catalyst may be tailored towards the particular organic pollutant which it is desired to destroy.
Advantageously, in the method according to the present invention, the catalyst does not dissolve in the aqueous solution. Since the integrity of the catalyst is maintained throughout the reaction, the catalytic activity is likewise maintained and fresh catalyst is not required to be added frequently. Also, the problems in homogeneous catalytic systems related to dissolved catalyst being washed away with the effluent are minimised.
The method according to the present invention is also highly efficient. It has been found that in excess of 99.9% of an organic compound in an aqueous effluent may be destroyed.
The organic compound may be present in a concentration above or below its solubility limit in water.
In contrast to some prior art methods of destroying organic compounds, the oxidation reaction in the present invention may be carried out at relatively low temperatures. The reaction may be carried out below 100xc2x0 C. Preferably the reaction is carried out below 50xc2x0 C. to avoid the problems associated with heating large volumes of water. The inventors have found that the reaction is efficient even when carried out at ambient temperature. Since the cost of heating large volumes of effluent can be very high in practice, the high efficiency of the method according to the present invention at ambient temperatures provides a significant advantage for industrial scale applications.
The oxidant employed in the reaction may be a commonly used oxidant such as hydrogen peroxide. The inventors have also found that air or oxygen is an efficient oxidant in the present invention. Both hydrogen peroxide and air have the advantage that they decompose to environmentally benign products. It is envisaged that many other oxidants may be used equally. For example, KMnO4 or ozone may be used. Other oxidants such as H2O2 and sodium hypochlorite mixtures, sodium peroxydisulphate and calcium hypochlorite may be efficient oxidants but may present their own environmental hazards and for that reason may be less favoured.
It will be appreciated immediately that the ease and low cost of being able to use air as an oxidant and operate at ambient temperature represents a considerable advantage.
The invention may be carried out as either a batch or continuous flow process.
The types of organic compounds which may be destroyed in the method according to the present invention include alkanes, alkenes, alkynes, aromatics, alcohols, aldehydes, ketones, carboxylic acids, esters, ethers, amines, detergents, organophosphates and derivatives of all these including compounds containing substituent groups and heteroatoms. In particular, the inventors have found that alkyl phosphates such as tributyl phosphate (TBP), sodium di-butylphosphate (NaDBP) and hydrogen monobutyl phosphate (HMPB) are efficiently destroyed. It should be understood that the foregoing list is not exhaustive and does not limit the invention in any way. In principle, all types of organic compound may be destroyed by the method according to the present invention. Several different organic compounds present in the same aqueous effluent may be treated together.
Particular industries each have their own array of problem compounds to deal with. The present invention is potentially applicable to all types of organic compound. In the nuclear industry for example, outflows from uranium processing facilities may contain a number of contaminant organic compounds such as tributyl phosphate (TBP), odourless kerosene (OK), ethylenediaminetetraacetic acid (EDTA) and citric acid. The present invention may be effective in destroying all of these compounds in aqueous solution, particularly at trace concentrations.